A level restoration circuit in a transimpedance amplifier removes the DC component, i.e. the average value which carries no information, of an optical signal exiting an optical fiber onto a photodiode, while at the same time keeping the low frequency −3 dB frequency low enough to meet requirements for both Telecom and Datacom applications.
With reference to FIG. 1, a conventional TIA circuit, generally indicated at 1, converts the current IPD exiting a photodiode 2, into an output voltage VOUT. The photodiode current IPD, which enters the TIA circuit 1 at an input terminal 3, includes both a DC component and an AC component. The AC component, which carries the information, must be maintained and sent down an amplification chain 4 to final receiving equipment (not shown), while the DC component should be ignored and if possible eliminated. A feedback circuit, generally indicated at 5, removes the DC component by means of negative feedback, implemented by a feedback amplifier 6/low pass filter (i.e. Capacitor 7) and a bypass transistor 8 combination. The feedback amplifier 6/low pass filter 7 has gain, and removes the AC component of a voltage feedback signal VFB, leaving only a DC component VFBDC. The capacitor 7 is used to set the low-frequency cutoff that the TIA circuit 1 requires. The bypass transistor 8 takes that DC component VFBDC of the voltage feedback signal VFB and generates a DC current IFBDC in the collector 9, which by the action of negative feedback equals the incoming DC current IPDDC from the photodiode 2. Accordingly, the DC component IPDDC is removed from the incoming signal IPD and passed to the ground GRND through the emitter 11 of the bypass transistor 8.
Unfortunately, the low frequency −3 dB cut off frequency, i.e. the low frequency cut off, of the TIA 1 can vary dramatically depending on the input current IPD, which makes meeting performance requirements difficult. The low frequency cut off of the entire wideband TIA 1 is proportional to the gain of the feedback circuit 5, as well as the size of the filtering capacitor 7. The gain of the feedback circuit 5 is a transconductance because the feedback circuit 5 samples the differential output voltage, Vout=Outp−Outm, and produces a current IFBDC at the collector 9 of the bypass transistor 8. The transconductance gain of the bypass transistor 9, according to basic small signal transistor theory is the collector current divided by the thermal voltage (GT=Ic/Vt=IFBDC/Vt), i.e. the gain varies with the DC current IFBDC flowing in the device. Accordingly, since the transconductance gain of the whole feedback circuit 5 is proportional to the transconductance gain of the bypass transistor 8, the low frequency cut off is proportional to the transconductance of the bypass transistor 8. This can cause a large variation in the low frequency cut off, since the photodiode current IPD can vary over a large range of values. For example: the photodiode DC current IPD can vary from about 10 uA, up to about 1 mA, which makes a 40 dB of difference between the low and high values of the transconductance gain of the bypass transistor 8, i.e. 20*log(1 mA/10 uA). Accordingly, the variation in transconductance gain causes the low frequency cut off to also vary significantly, i.e. if the low frequency cut off is set to 50 kHz at a low input current, the low frequency cut off could get as high as 5 MHz at a high input current, which is quite unacceptable in many applications including Ethernet and Sonet Telecom. Moreover, the filter capacitor 7 must be designed to be large enough to maintain the low frequency cut off small, which can result in unreasonably large capacitors affecting the size of the required packaging.
U.S. Pat. No. 6,404,281, issued Jun. 11, 2002 in the name of Kobayashi et al; U.S. Pat. No. 6,504,429, issued Jan. 7, 2003 to Kobayashi et al; and U.S. Pat. No. 6,771,132 issued Aug. 3, 2004 to Denoyer et al disclose improvements to TIA feedback circuits that include minimizing the upper limit of the low frequency cut off frequency; however, none of these references addresses the problem caused by the variation in transconductance gain.
An object of the present invention is to overcome the shortcomings of the prior art by providing a feedback circuit with a relatively small transconductance gain variation resulting in relatively small variation in the low frequency cut off frequency over a range of photodiode input currents.